The present invention relates to a phosphor which emits light by the excitation of X-rays and a X-ray intensifing screen using the phosphor, and relates mainly to a X-ray intensifing screen used for medical and industrial radiography.
A phosphor which emits light by the excitation of X-rays is used mainly for X-ray intensifing screen. A X-ray intensifing screen is generally used in combination with photographic film for the purpose of improving the sensitivity of the photographic system for medical radiography. The phosphor used for such a X-ray intensifing screen is required to have a high X-ray dose, a high light emission efficiency, and a small afterglow component.
A X-ray intensifing screen coated with phosphor of a high X-ray dose has an excellent granularity of X-ray image and therefore can improve the diagnosis efficiency in medical radiography. A phosphor having a high light emission efficiency can be used with only a small quantity of X-ray irradiation, and thereby the exposure dose of the subject can be reduced. Further, a phosphor having a low quantity of afterglow component can prevents errors in consultation caused by after image (afterglow noise).
Recently, based on the desire for reducing exposure dose of the subjects, X-ray intensifing screens using a phosphor containing Gd.sub.2 O.sub.2 S:Tb, BaFCl:Eu, LaOBr:Tm, YTaO.sub.4 :Tm or the like, instead of conventional phosphor of CaWO.sub.4 have been put to practical use.
However, since BaFCl:Eu and LaOBr:Tm have low X-ray absorption dose, the granularity of the obtained X-ray photograph is not good. Further, since the phosphor is shaped into flat granules, light emitted by the excitation of X-rays is much scattered and the sharpness of the X-ray image becomes low.
A phosphor containing Gd.sub.2 O.sub.2 S:Tb emits light in the range from the blue zone to the green zone, and is used in combination with an orthochromatic film which is sensitive in the range from the blue zone to the green zone. Therefore, the film is easily exposed to light even in the darkroom and the darkroom lamp is required to be dim, which causes poor workability.
A phosphor containing YTaO.sub.4 :Tm has a high afterglow component, and afterglow noises occur during continuous photographing, which limits its use. If a phosphor is developed which has a small afterglow component and is provided with the advantages of YTaO.sub.4 :Tm phosphor, an ideal phosphor for X-ray use can be realized.